1. Field of the Invention
The invention relates to ablative high-temperature elastomers. More particularly, this invention relates to dual-cure silphenylene-methylvinylsiloxane polymers.
2. Description of the Prior Art
Methylphenylsiloxane polymer binders containing fillers of silica, silicon carbide, and carbon fiber are useful as char-forming ablative high temperature insulators. Under certain types of severe heating these insulators undergo blistering and spalling. Blistering and spalling can be avoided by using ablative insulators containing polymer binders of greater thermal stability. Fluorocarbon elastomers such as fluorocarbon-ether-linked polyheterocyclic elastomers have greater thermal stability than methylphenylsiloxane polymers but fluorocarbons are not char-forming ablators.
Silphenylene-siloxane copolymers have greater thermal stability than methylphenylsiloxane polymers as well as suitable char-forming properties. However, to achieve precure viscosity sufficiently low to produce flowable materials when fillers and fibers are added, a relative short chain length polymer is required. The short chain length polymer is detrimental to good mechanical properties required for erosive flow resistance. Modifications of silphenylene-siloxane copolymer mechanical properties are needed before the copolymers can be useful in char-forming ablative high temperature insulators.
Modification of organopolysiloxane compositions with polyurethane is a known method of improving mechanical properties. Block copolymers comprising polysiloxane and urethane segments are known. Polysiloxane mixtures filled with polyurethane are also known. These mixtures are characterized as having a coherent phase of an organopolysiloxane liquid and a non-coherent, discernable phase of polyurethane polymer, in some cases with partial chemical and/or physical bonding to one another. Both the polysiloxane/polyurethane block copolymers and the polyurethane filled polysiloxane have excellent elastic properties but poor thermal stability. The various polymers and processes of the prior art do not enable the modification of silphenylene-siloxane copolymers to improve the mechanical properties without compromising the desired high thermal stability.